In conventional metal working processes, metallic workpieces, e.g. casting or forgings, may be encapsulated in low melting metals or metallic alloys to provide a convenient means for fixturing the workpiece during the subsequent machining operations. Low melting alloy encapsulants are well known in the art, are commercially available under a variety of trade names, e.g., Cerrobend, Cerrotru, and typically comprise combinations of such metals as bismuth, lead, tin, cadmium, antimony and zinc. Any residual encapsulant remaining after the machining operations are completed is typically removed by immersing the encapsulated workpiece in a hot oil bath to melt the encapsulant.
In certain demanding applications, e.g. gas turbine engine airfoils, the presence of even trace amounts of residual low melting alloy may have severe negative impact upon the high temperature properties of the workpiece. Fastidious cleaning and scrupulous inspection are required to ensure complete removal of the encapsulant. When a high level of cleanliness is critical, secondary cleaning steps are typically included in the removal process. Removal is particularly difficult in the case of workpieces having hidden surface area, e.g., airfoils having internal cooling passages.
In an exemplary process, the removal of the bulk of the low melting alloy from a gas turbine engine blade by melting is followed by immersion of the gas turbine engine blade in a strong acid solution to dissolve the residual low melting alloy. The gas turbine engine blades are then rinsed and individually immersed in separate aliquots of cleaning solution. After a period of time, each aliquot is analyzed by atomic absorption spectroscopy to detect the presence of dissolved low melting alloy contaminants in the aliquot. Gas turbine engine blades are recycled through the cleaning and testing steps until the concentration of dissolved contaminants has dropped below a preselected maximum concentration. The process is very time consuming and highly labor and capital intensive.